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SC623EVB Datasheet(PDF) 11 Page - Semtech Corporation |
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SC623EVB Datasheet(HTML) 11 Page - Semtech Corporation |
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11 / 21 page  SC623 11 General Description This design is optimized for handheld applications supplied from a single Li-Ion cell and includes the following key features: A high efficiency fractional charge pump that supplies power to all LEDs Four matched current sinks that control LED backlighting current, with 0.5mA to 25mA per LED An LED flash output that provides up to 400mA of momentary current or up to 250mA of continuous spotlight current A SemWire single wire interface that provides control of all device functions High Current Fractional Charge Pump The backlight and flash outputs are supported by a high efficiency, high current fractional charge pump output at the VOUT pin. The charge pump multiplies the input voltage by 1, 1.5, or 2 times. The charge pump switches at a fixed frequency of 250kHz in 1.5x and 2x modes and is disabled in 1x mode to save power and improve efficiency. The mode selection circuit automatically selects the 1x, 1.5x or 2x mode based on circuit conditions. Circuit conditions such as low input voltage, high output current, or high LED voltage place a higher demand on the charge pump output. A higher numerical mode may be needed momentarily to maintain regulation at the VOUT pin during intervals of high demand, such as the high current of an LED flash or the droop at the VIN pin during a supply voltage transient. The charge pump responds to these momentary high demands, setting the charge pump to the optimum mode (1x, 1.5x or 2x), as needed to deliver the output voltage and load current while optimizing efficiency. Hysteresis is provided to prevent mode toggling. The charge pump requires two bucket capacitors for low ripple operation. One capacitor must be connected between the C1+ and C1- pins and the other must be • • • • connected between the C2+ and C2- pins as shown in the typical application circuit diagram. These capacitors should be equal in value, with a minimum capacitance of 2.2μF to support the charge pump current requirements. The device also requires a 2.2μF capacitor on the VIN pin and a 4.7μF capacitor on the VOUT pin to minimize noise and support the output drive requirements. Capacitors with X7R or X5R ceramic dielectric are strongly recommended for their low ESR and superior temperature and voltage characteristics. Y5V capacitors should not be used as their temperature coefficients make them unsuitable for this application. LED Backlight Current Sinks The backlight current is set via the SemWire interface. The current is regulated to one of 32 values between 0.5mA and 25mA. The step size varies depending upon the current setting. Between 0.5mA and 12mA, the step size is 0.5mA. The step size increases to 1mA for settings between 12mA and 15mA and 2mA for settings greater than 15mA. This feature allows finer adjustment for dimming functions in the low current setting range and coarse adjustment at higher current settings where small current changes are not visibly noticeable in LED brightness. All backlight current sinks have matched currents, even when there is variation in the forward voltages (ΔV F ) of the LEDs. A ΔV F of 1.2V is supported when the input voltage is at 3.0V. Higher ΔV F LED mis-match is supported when V IN is higher than 3.0V. All current sink outputs are compared and the lowest output is used for setting the voltage regulation at the VOUT pin. This is done to ensure that sufficient bias exists for all LEDs, including the flash LED. The backlight LEDs default to the off state upon power- up. For backlight applications using less than four LEDs, any unused output must be left open and the unused LED driver must remain disabled. When writing to the Backlight Enable Control register, a zero (0) must be written to the corresponding bit of any unused output. Applications Information |
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Similar Description - SC623EVB |
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